1. Field of Invention
This invention relates to integrated circuit band gap reference sources, specifically, to a design that requires only one polarity of bipolar transistor and does not require any resistors.
2. Description of Prior Art
Numerous patents have been issued for band gap reference voltages, the most basic implementation of which is shown in FIG. 1. This design is well known and will be reviewed here only very briefly. In designs of this type, a differential base-emitter voltage drop is generated across a pair of bipolar junctions, and this difference is amplified by a resistor ratio. The resultant voltage has a positive temperature coefficient. This resultant voltage is then added to a junction voltage, which has a negative temperature voltage. By well known means the positive temperature coefficient is set to equal the negative temperature coefficient, and the result is a voltage with very small temperature variation.
A more recent example based on the principles of the design of FIG. 1 using a multiplicity of junctions on each side of the amplifier is found in U.S. Pat. No. RE35,951.
Because the initial differential voltage drop is amplified by the resistor f ratio, any noise associated with the voltage, including noise from the circuits which bias them up, is similarly amplified.
A recent U.S. Pat. No. 6,288,525, does not require any resistors. In this design, noise is the root mean square sum of the noise of multiple junctions, rather than the amplification of a few junctions, so output noise is less than that of earlier designs. However, this design requires both PNP and NPN transistors. In standard lowest cost CMOS processes, PNP transistors with the collector tied to the substrate require no special processing. To obtain an NPN transistor, a BiCMOS process is typically used, increasing cost.
In a recent publication (IEEE Journal of Solid State Circuits, January 2002, page 81-83), another design is reported that does not need resistors. However, the basic design makes use of a small differential voltage drop amplified by ratios of MOS transistors. This approach requires an inverse current function based on long channel MOS transistor theory. The differential voltage is converted to a current, amplified by MOS transistor rations, and converted back to a voltage. Any noise associated with the transistors used to develop the differential voltage, including noise from the circuits which bias them up, is similarly amplified.
Another recent U.S. Pat. No., 6,614,209, also does not require resistors. However, this design is based on using a cascade of proportional to absolute temperature (PTAT) voltage generators, each having its own differential amplifier. Each differential amplifier adds to the die area and to the total noise.
Trimming a band gap reference is typically done by adjusting a resistor. Such trimming requires substantial die area for good matching. Furthermore, the fusible links used to control the trimming typically are used to turn MOSFET switches on or off. These MOSFET switches are typically in series with resistors that form series and parallel connections of resistors. Consequently, the on resistance of the switch must be very small with respect to the resistors so that the resistance of the switch can be ignored. This requires physically large switches, using considerable die area. An alternative method buffers the output voltage with an operational amplifier and trims the gain of the amplifier. While this does not require large switches, it does require additional resistors and the amplifier. An alternate method of trimming is disclosed in US Patent Application Publication 2002/0070793, and this method can be applied to the invention described herein.